Method for preparing complementary semiconductor device

ABSTRACT

A complementary semiconductor device includes P-and N-type semiconductor regions separately formed in a semiconductor substrate and having substantially the same concentration of impurities. N-and P-channel type silicon gate field effect transistors are formed in the P-and N-channel type regions, respectively. Gate electrodes of the P-and N-channel type silicon gate field effect transistors are formed by polycrystalline silicons of the same conductivity type. An impurity of the same conductivity type is doped into both the semiconductor regions to provide channel doped regions.

This is a division of application Ser. No. 922,192 filed July 5, 1978now U.S. Pat. No. 4,209,797.

BACKGROUND OF THE INVENTION I. Field of the Invention

This invention relates to a semiconductor device and a complementarytype silicon gate field effect semiconductor in which P- and N-channeltype silicon gate field effect transistors are formed on the samesubstrate.

II. Description of the Prior Art

In a complementary type silicon gate field effect semiconductorintegrated circuit device (hereinafter referred to as a silicon gateCMOS.IC), the threshold voltages of P channel and N channel type fieldeffect transistors are so set as to satisty the requirements as to thethreshold voltage of the circuit, operation speed, power consumptionetc. Usually, the threshold voltages of the transistors are lowered to arange of 0.8 to 2.0 volts within which the low power consumptionproperty of the CMOS.IC, is not affected.

An impurity of predetermined conductivity type is doped into the channelregion (i.e. channel doping) below the gate electrode of each transistorto control the impurity concentration of the channel region of eachtransistor. In this way, the threshold voltages of transistors arecontrolled to a desired value. For example, an island-like P-type regionis conventionally formed in an N-type silicon substrate and an impurity,such as phosphorus, is doped by channel doping in that portion of anisland-like P-type region and of N-type silicon substrate where achannel region of the transistor is formed. In this way, channel dopedlayers are formed. Then, an impurity, such as boron, is doped in theN-type silicon substrate to form source and drain regions. In this case,a gate oxide film on that portion of the semiconductor structurecorresponding to the channel region and polycrystalline silicon layeroverlying the gate oxide film are used as a mask for diffusion. Thus, aP-channel type silicon gate field effect transistor is formed. Likewise,an impurity such as phosphorus is doped into the island-like P-typeregion with the gate oxide film and polycrystalline silicon layer as amask to form source and drain regions. Thus, an N-channel type silicongate field effect transistor is formed.

If the thickness of the gate oxide film, fixed positive charge, andimpurity concentration of the polycrystalline silicon layer aredetermined, then the threshold voltage of the respective transistors isunconditionally determined by the impurity concentration of regions(i.e. island-like P- and N-type regions) in which each transistor isformed. Since the channel doped layer is formed to shift the thresholdvoltage of the respective transistors to a desired level, impurities ata specified concentration are doped in the channel region. It isadvantageous from the standpoint of manufacturing considerations tosimultaneously form channel doped layers, but the conventional methodhas the following disadvantages.

Suppose that the threshold voltage of each transistor is controlled to areasonable value of, for example ±1.0 volts. If in this case thethreshold voltage of both P- and N-channel transistors are to besimultaneously controlled by a one channel doping operation, it isnecessary that the impurity concentration of a N-type silicon substrate(i.e. a substrate region of a P-channel type silicon gate field effecttransistor) be, for example, about 2×10¹⁵ atoms/cm³ and that theimpurity concentration of an island-like P-type region (i.e. substrateregion of an N-channel type silicon gate field effect transistor) be ahigher value of, for example, about 1×10¹⁶ atoms/cm³. Otherwise it isimpossible to simultaneously control the threshold voltages of bothtransistors. This fact can be seen from FIG. 1 which shows therelationship between the threshold voltage of each transistor and theimpurity concentration of the regions in which each transistor isformed.

Since in this way the impurity concentration of the substrate becomeshigher, the junction capacitance of source and drain regions of thetransistor, as well as the substrate bias effect becomes great. If arandom access memory, for example, is constructed using such silicongate CMOS.IC, a slower access time is involved.

Furthermore, since the polycrystalline silicon layer which serves as amask in the formation of source and drain regions of each transistor isdoped with an impurity of different conductivity type, a direct mutualconnection of the polycrystalline silicon layers can not be effected andmust be made through an aluminum connection, thus lowering anintegration density.

SUMMARY OF THE INVENTION

An object of this invention is to provide a complementary semiconductordevice in which the values of the threshold voltages of N- and P-channeltype silicon gate field effect transistors can be controlled to a propervalue and the junction capacitance of the N-channel type silicon gatefield effect transistor, as well as the substrate bias effect, can bedecreased.

Another object of this invention is to provide a complementarysemiconductor device capable of attaining high integration density.

These and other objects which will be apparent from the followingdetailed description have been attained according to this invention by acomplementary semiconductor device comprising: a semiconductor substrateof one conductivity type having a low impurity concentration; P- andN-type semiconductor regions having substantially the same impurityconcentration; an N-channel type silicon gate field effect transistorincluding source and drain regions in the P-type semiconductor regionand a gate region including a gate electrode constituted by apolycrystalline silicon layer; a P-channel type silicon gate fieldeffect transistor including source and drain regions in the N-typesemiconductor region and a gate region including a gate electrodecomprising a polycrystalline silicon layer, said polycrystalline siliconlayer having the same conductivity type as that of the N-channel typetransistor; a first impurity layer formed in that portion of the P-typesemiconductor region corresponding to the gate region and determiningthe threshold voltage of said N-channel transistor; and a secondimpurity layer formed in that portion of the N-type semiconductor regioncorresponding to the gate region and determining the threshold voltageof said P-channel transistor, said second impurity layer being formed bydoping the same conductivity type impurity as that of said firstimpurity layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing a relation of the threshold voltage of eachtransistor to an impurity concentration of a substrate region in a priorart complementary semiconductor device;

FIGS. 2A to 2J are schematic views showing a process for the manufactureof a complementary semiconductor device according to this invention;

FIGS. 3 and 4 are graphs, each, showing a relation of the thresholdvoltage of each transistor to an impurity concentration of a substratein a complementary semiconductor device according to this invention;

FIG. 5 shows an equivalent circuit showing a silicon gate CMOS randomaccess memory based on the complementary semiconductor device accordingto this invention; and

FIG. 6 is a schematic view showing a complementary semiconductor deviceaccording to another embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of this invention will be explained by referring to theaccompanying drawings in which like reference numerals are employed todesignate like parts or elements.

Explanation will now be given to a method for the manufacture of acomplementary type semiconductor device, by referring to FIGS. 2Athrough 2J.

A thin, thermally oxidized film (oxide film) 12 is formed, as shown inFIG. 2A, on the whole surface of an N-type silicon substrate having alow impurity concentration, i.e., about 10¹³ to 10¹⁴ atoms/cm³. Aphotoresist film 14 is formed on the film 12, and an opening 16 isprovided in a predetermined portion of the photoresist film 14. With thephotoresist film 14 used as a mask for diffusion, phosphorus is doped,by an ion implantation method, through the opening 16 to form anisland-like, N-type semiconductor region (N type well) 18 having animpurity concentration of the order of 10¹⁵ atoms/cm³ (usually about2×10¹⁵ atoms/cm³).

After the photoresist film 14 is removed, a new photoresist film 20 isformed, as shown in FIG. 2B, on the resultant semiconductor structureand an opening 22 is formed in the photoresist film 20. Likewise, boronis doped by the ion implantation method into the semiconductor structureto form an island-like P-type semiconductor region (P-type well) 24. TheP-type well 24 is spaced apart from the above-mentioned N-type well 18.

After the photoresist film 20 and oxide film 12 are removed, the wholeexposed surface of the semiconductor structure is thermally oxidized toform a relatively thick (usually about 1 μm) field oxide film 26. Thefield oxide film 26 is removed to expose, as shown in FIG. 2C, thosesubstrate surface portions corresponding to P- and N-channel type fieldeffect transistors to be formed respectively in the N- and P-type wells18 and 24, as well as that substrate surface portion corresponding to aPN junction, or its neighborbood of the P-type well 24.

As shown in FIG. 2D, a thermally oxidized film (oxide film) 28 of about1000 to 1500 A (for example 1200 A) is formed on the exposed substratesurface portions. In order to control threshold voltages of transistorsto be formed to a proper level as will be described later in detail, Pconductivity type impurity such as boron is doped by the ionimplantation in those portions corresponding to N- and P-channels of thesubstrate to simultaneonsly form impurity layers 30 and 32. Since theamount of impurity doped is sufficiently smaller than that doped in theformation of source and drain regions to be later formed, it does notmatter if such impurity layer is formed on other than a channel region,i.e. in the source and drain regions. Also such impurity layer is formedin the PN junction area as shown at 31.

As shown in FIG. 2E a polycrystalline silicon layer 34 with an impurity(phosphorus) concentration of about 1×10¹⁹ atoms/cm³ is formed on theoxide films 26 and 28 and a low-temperature oxide film 36 is formed onthe whole surface of the resultant semiconductor structure. Then, aphotoresist layer is formed on the oxide film 36 and it is removedexcept for those portions 38 which correspond to gate regions of the P-and N-channel FET's.

With the photoresist film 38 as a mark, etching is effected to removethe low-temperature oxide film except for those portions under theresist film 38. The polycrystalline silicon layer except for thoseportions under the remaining low-temperature oxide films 36a is removedby plasma etching (FIG. 2F). Then, the oxide film 28, except for thoseportions 28a under the remaining polycrystalline layer 34, is removed byetching to expose those substrate surface portions corresponding to thesource and drain regions in each of the N- and P-type wells 18 and 24,respectively, as well as that substrate surface portion corresponding tothe PN junction area of the P-type well 24. The photoresist layer 38 isetched away (FIG. 2G).

A phosphorus bearing silicon oxide film (PSG) 40 and low-temperatureoxide film 42 are formed, in that order, on the whole surface of theresultant semiconductor structure. Then, the PSG film 40 andlow-temperature oxide film except for those portions corresponding tothe PN junction area of the P-type well 24 are removed as shown in FIG.2H. With the oxide films 26, 28a and 36a and polycrystalline siliconlayer 34a as a mask, boron is diffused in the vapor phase into theN-type well to form P-type source and drain regions 44 and 46 having animpurity concentration of about 1×10¹⁹ atoms/cm³ and a depth of 1 to 2μm. At the same time, with the oxide films 26, 28a and 36a andpolycrystalline silicon layer 34a as a mask, phosphorus is diffused fromthe PSG film 40 into the P-type well to form N-type source and drainregions 50 and 48 having an impurity concentration of about 1×10¹⁹atoms/cm³ and a depth substantially equal to that of the P-type sourceand drain regions 44 and 46. At this time, an N-type region 52 is formedin the PN junction area of the P-type well 24. Since the concentrationof the impurity doped in the formation of the source and drain regionsis much higher than that of the impurity layers 30 and 32 and ofimpurity layer 31 formed in the PN junction area of the P-type well 24,semiconductor regions of desired conductivity type are formed withoutbeing influenced by the concentration of these impurity layers. In thiscase, the impurity layer portions under the oxide film 28a are left aschannel doped layers 30a and 32a. Then, the oxide films 36a and 42 andPSG film 40 are etched away to provide a semiconductor structure asshown in FIG. 2I.

A low-temperature oxide film 54 is formed on the whole surface of thesemiconductor structure of FIG. 2I, and the source regions 44, 50 anddrain regions 46, 48 of the transistors are partially exposed. Aluminum,for example, is evaporated on the exposed surface of the semiconductorstructure to provide source electrodes 56, 6 and drain electrodes 58, 62of the transistors. The drain electrodes 58 and 62 are connected by analuminum connection 64. A gate connection electrode is formedsimultaneously with the formation of the source and drain electrodes. Inthis case, the polycrystalline silicon layer 34a is beforehand extendedoutside of the transistor region and the gate connection electrode isformed at the extension of the silicon layer 34a. A passivation layer 66is formed on the surface of the semiconductor structure to obtain asilicon gate CMOS.IC as shown in FIG. 2J. Now, explanation will be givento the formation of the channel doped layers 30 and 32 as explained inconnection with FIG. 2D.

Suppose that in the above-mentioned semiconductor device the gate oxidefilm 28a has a thickness (Tox) of 1200 A, each polycrystalline siliconlayer 34a of the same N conductivity type has an impurity concentrationof 1×10¹⁹ atoms/cm³, and that the fixed positive charge density, Nss, is5×10¹⁰ /cm². In this case, a relation of the threshold voltage of eachtransistor to the impurity concentration of the N- and P-type wells isas shown in FIG. 3, provided that such channel doped layer 30 an 32 arenot formed. Because the impurity concentration of the N- and P-typewells is 2×10¹⁵ atoms/cm³, the threshold voltages of the N- andP-channel tansistors are +0.25 volt and -1.75 volts, respectively (seeFIG. 3). If the absolute values of the threshold voltages of both thetransistors are controlled to have a proper value of, for example,|1.0|V, all that is necessary is to shift the threshold voltages of boththe transistors by 0.75 volt in the positive direction.

In consequence it is only necessary that a dose of boron of about9.5×10¹⁰ atoms/cm², i.e. an amount required to shift a threshold voltageof 0.75 volt, be doped at an acceleration voltage of about 55 KeV in theformation of the channel doped layers 30 and 32.

According to this invention, the N-type silicon substrate has a lowconcentration of about 10¹³ to 10¹⁴ atoms/cm³ and, by so doing, theP-type well which provides a N-channel type silicon gate FET substrateregion is formed to have a low concentration of about 2×10¹⁵ atoms/cm³.If the source and drain regions are formed at a dimension of about 30μm×20 μm, the junction capacitance of the source and drain regions ofthe N-channel type silicon gate FET is about 0.085 PF, a value about1/2.3 times as low as about 0.19 PF (prior art). The substrate biaseffect is also dependent upon the concentration of the P-typesemiconductor layer and it is very much smaller than that of theconventional semiconductor device. Where a CMOS circuit such as aninverter circuit, shift register circuit, counter circuit etc. is formedusing the semiconductor device of this invention, the operating speed isfaster than that of the conventional counterpart and power dissipationresulting from charging and discharging currents becomes very small. ACMOS device of this invention can be advantageously applied to a largebits silicon gate CMOS random access memory. Such a memory has as abasic unit a memory cell unit as shown in FIG. 5. The memory cell unitincludes a memory cell Mc including P-channel type silicon gate FET'sQ₁, Q₂ and N-channel type silicon gate FET's Q₄, Q₅. A pair of switchingtype silicon gate FET's Q₃ and Q₆ are connected one between the memorycell Mc and a digit line D and the other between the memory cell Mc anda digit line D so as to be opened and closed by a bit signal which isapplied to the bit line B. Transistors Q₃ and Q₆ are preferably made ofN-channel types which are faster in switching speed than P-channeltypes.

Capacitances CD and CD connected to the digit lines D and D include thejunction capacitance of the switching type N-channel type silicon gateFET, gate capacitance of the next stage transistor, capacitance causedby an Al or poly-silicon lead etc. The capacitances CD and CD aresubstantially determined by the junction capacitance of the N-channeltype silicon gate FET for switching.

Where, as in a 4K-bit random access memory, for example, 64 N-channeltype silicon gate field effect transistors Q₃ and Q₆ are connected tothe digit lines D and D, the capacitances CD and CD become a sum of thejunction capacitances of the transistors Q₃ and Q₆ and, when theconventional CMOS transistor is used, the capacitance will be about 5.4PF. For the CMOS transistor of this invention, the capacitance is a verysmall value of about 1.8 PF. Since the substrate bias effect of theN-channel type silicon gate FET becomes small as already set out above,a very fast access time is obtained.

According to this invention the silicon gate electrodes of the N- andP-channel type silicon gate FET's are of the same conductivity type.Where the gate electrodes of the N- and P-channel type silicon gateFET's are connected in common and the common connection is used as aninput terminal, it is necessary in the conventional method to connecttogether the gate electrodes through an Al connection and thus a greaterarea is required to provide a contact portion. According to thisinvention, on the other hand, a high integration density can beattained, because it is not necessary to connect together the gateelectrodes through the Al connection: they can be directly connectedtogether.

Although in the above-mentioned embodiment the silicon gate electrode ofeach transistor is a phosphorus-containing N-type, it can be aboron-containing P-type with the same impurity concentration (i.e.1×10¹⁹ atoms/cm³). In this case, a relation of the threshold voltage ofeach transistor to the impurity concentration of the N- and P-type wellswill be as shown in FIG. 4, provided that the other conditions areidentical. When the impurity concentration of each well is 2×10¹⁵atoms/cm³, the threshold voltages of the N- and P-channel type FET's areabout +1.25 volts and about -0.75 volt, respectively. In order to letthe absolute values of the threshold voltages of both the transistors tobe controlled to a proper, mutually equal value of |1.0|, a dose ofphosphorus of about 9×10¹⁰ atoms/cm², i.e. an amount necessary to shiftthreshold voltage by 0.25 volt in the same negative direction, is dopedat the acceleration voltage of about 130 KeV to simultaneously formchannel doped layers.

Although in the above-mentioned embodiment the channel doped layer isformed after the gate oxide film has been formed, it can be formedbefore the formation of the gate oxide film. While the polycrystallinesilicon is subjected to patterning using the plasma etching method,patterning can be freely effected by an ordinary etching method with anitride film as a mask. This invention can also be applied to asemiconductor thin film on an insulating substrate such as sapphire,spinel etc.

FIG. 5 shows an embodiment in which N- and P-type semiconductor regions18 and 24 are separately formed by an insulator 102 on an insulatingsubstrate 100 such as sapphire, spinel etc. and a predetermined silicongate FET is formed in each of the semiconductor regions 18 and 24.

What we claim is:
 1. A method for manufacturing a complementarysemiconductor device comprising:forming P- and N-type semiconductorregions in a semiconductor substrate of one conductivity type having alow impurity concentration; forming impurity layers by doping the sameconductivity type impurity into predetermined portions of said P- andN-type semiconductor regions; forming gate regions to partially covereach of the impurity layers, said gate regions including polycrystallinesilicon layers of the same conductivity type; and simultaneously formingsource and drain regions in the respective P- and N-type semiconductorregions to form N- and P-channel type field effect transistors whereinsaid impurity layer is formed by doping that amount of impurity which isenough to control the threshold voltages of the N- and P-channel typetransistors to a desired level.